Cross-hole sonic logging (CSL) is commonly used for the evaluation of the structural integrity of drilled shafts. Cross-hole sonic tomography (CST) is also performed to generate a more detailed image of the suspected defect detected by CSL. CST is more demanding than CSL in terms of both data acquisition and analysis. However, it is reported in the literature that these methods exhibit some limitations in detecting defects of certain geometry within drilled shafts. Recently the full waveform inversion (FWI) technique in non-destructive testing applications has shown promising performance in providing high resolution images of the domain of interest. FWI is implemented by solving an iterative optimization problem that simulates wave propagation through the domain. The choice of starting model is of significant importance to ensure the convergence of the inverted model to a physically meaningful result. In this study the application of FWI as an alternative to CST is investigated through numerical simulations. The focus is primarily on improving the convergence of the inversion by examining a hybrid combination of misfit functions over a range of frequencies for the modeled waveforms. In addition to the choice of the misfit function, this study will investigate the performance of a multi-scale FWI for structural integrity of drilled shafts.
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